1. Field of Invention
The present invention relates to an installation for increasing a scanning range of a scanner. More particularly, the present invention relates to an installation for increasing a scanning range of a scanner along an axial direction of a light source.
2. Description of Related Art
In general, a scanning system can be roughly classified as a reflective type or a light-penetrating type. In the reflective scanning system, a document is placed over a transparent glass panel. Light from a light source travels through the transparent glass panel and hits the document. The light is reflected back from the document to reach an optical sensor after passing through a set of optical elements. The optical sensor, for example, can be a charge couple device (CCD) where a scan image is formed. In the light-passing scanning system, the document is also placed on top of a transparent glass panel. However, light from a light source has to pass through the document, the transparent glass panel and a set of optical elements before arriving at the optical sensor.
Hence, both the reflective and the transparent type of scanning system operate on similar principles. In the reflective scanning system, brightness variation of the reflected light from a document is gauged to form an image. On the other hand, in the transparency scanning system, brightness variation of light after passing through the document is gauged to form an image.
However, the brightness level along a conventional longitudinal lamp tube, in other words, along the light axis of a light source, is brightest in the middle and dimmer on either side. Hence, the image produced by the light source will also be brighter in the middle than either side. Since a scanner works according to variation in brightness level, capacity for detecting brightness contrast near the two end sections of the light axis is lower. If the variation in brightness level along the axial direction of the light source is large, quality of a scan image near the edges of the light axis may deteriorate. Therefore, variation of brightness level along the light axis of a light source is an important parameter in document scanning.
In brief, the lesser the variation of brightness level along the light axis, the better will be the scanning range of the light source and the effective contrast of the document.